G-Man's true purpose is unknown. He was hired in order to follow Gordon and watch what he does in the original Half-Life. He offers Gordon a job and places him in stasis. G-Man exists in order to represent that despite there appearing to be the illusion of choice in game, everything runs linear. He's there as a plot device to explain why it's so straightforward.

For a short while, (I played Half life 2 before the first, remember) I thought he was the man behind everything. Although after a short time, I realised that was totally wrong. But I think he's more of a... 'Guardian Angel' to Gordon, than anything else. He does drop little hints on it, I think. During Half Life 2 episodes. The episodes shed some light on him, but hopefully episode 3 tells us who he is.

Infact, I'd rather it didn't... I want the mystery to stay But I think a confrontation between Gordon and GMan would be win. Not just through the mind, like in the Half life 2 series, but like... A proper face-to-face meet.

At the end of half-life 1 G-man says that he has been watching you, and is very impressed. He offers you a contract (which Gordon signs by stepping into the portal). So he is--in a way--your boss.

Breen is also aware of this contract, by stating to Gordon that his contract was open to the highest bidder.
So if this is true, that must mean that there are bigger forces at play than just the Combine empire enslaving humanity; if the G-man hopes to profit off this whole ordeal.
If it is not true, then it is Breen trying to make you turn on the g-man, refusing to work(something like that).

He is there to piss you off at night while you worry yourself over question like this, and while you try and figure out who he is, he is here to cause us severe mental trauma from all the confusing links between him and random shit.

G-Man's true purpose is unknown. He was hired in order to follow Gordon and watch what he does in the original Half-Life. He offers Gordon a job and places him in stasis. G-Man exists in order to represent that despite there appearing to be the illusion of choice in game, everything runs linear. He's there as a plot device to explain why it's so straightforward.

Everything isn't linear. Play the intro of Episode One, and you'll know why.
Episode One starts of a short time BEFORE the end of Half-life 2.

G-man Has his own employers. Valve has even admitted that there is no evidence in any game (purposefully put in, ergo if anyone finds something that leads to gmans identity, its a misinterpretation.) But they have denied that it is gordon from the future, And he is more than human. Or a ninja. With magic. Hell, he could be a freakin god guiding gordon to help stop the combines interdimensional reign.

G-Man's true purpose is unknown. He was hired in order to follow Gordon and watch what he does in the original Half-Life. He offers Gordon a job and places him in stasis. G-Man exists in order to represent that despite there appearing to be the illusion of choice in game, everything runs linear. He's there as a plot device to explain why it's so straightforward.

It's funny how Gordon Freeman is "The Last Free Man" according to the rebels when in reality he has no freedom at all. He is more enslaved than any of the citizens.